The growth of the forest depends on a series of variables and their complex net of interactions. When the forest is composed of multiple species (called a “mixture”) its growth also depends on the intra- and inter-specific interactions between these species. Given the rising interest for biodiversity conservation, among other ecosystem services, there have been a number of growing studies on mixing effects. In this post I would like to quickly present the main results of my newly published paper on this topic.

Forests rich in tree species are considered to provide higher levels of ecosystem services and functions than single-species forests. According to many studies, mixed-species forests are also likely to be more resistant and resilient to climate change stressors and ecological disturbances. It has also been demonstrated that forests composed of many tree species can often be more productive than monocultures. However, some authors also showed that complementarity (or mixing) effects for any given tree species are hardly generalizable because they can vary greatly along gradients of climatic conditions and resource availability. Let’s make an example. Let’s say that it is well known that the growth of species A benefits from the presence of species B. However, when environmental or stand conditions change (e.g., higher temperature, higher stand density), species B becomes more competitive. Species A start growing less and loose the initially detected benefit. In this case, the positive complementarity effect may suddenly turn to negative.

READ MORE: Ecosystem services, mountain forests and climate change

There have been many studies on mixing effects but most of them focused on specific mixture types (e.g., beech-pine, pine-oak or spruce-fir). In the context of the project Swiss-SpeMixMod (“Integrating Species Mixtures in Tree Growth Functions for Forest Development Models in Switzerland”) funded in the framework of the COST Action EuMIXFOR, my colleagues and I at the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) set up a study to evaluate patterns of complementarity on individual-tree growth for the major central European tree species growing in a total of 19 mixture types. Our great advantage was the access to the entire Swiss National Forest Inventory (NFI). The Swiss NFI is based on terrestrial sampling on a 1.4×1.4 km grid of permanent plots covering the entire Switzerland. Since the first survey (1983-85), a total of three campaigns have been completed (NFI1, NFI2, NFI3). The fourth inventory began in 2009 as a continuous survey and will be completed by the end of this year. A fifth inventory will follow right after. The Swiss NFI, as any other country-level NFI, is a great source of data with the strength of being representative for a very broad range of environmental conditions, stand development, stand densities, forest types, silvicultural regimes and species compositions.

READ MORE: The response of the forest to drought: the role of stand density and species diversity 

We examined individual-tree growth in multiple mixtures at the level of species identity. This allowed us to evaluate whether patterns of complementarity for a given species change depending on the composition of the stand (not only at increasing number of species). We took advantage of state-of-the-art non-linear mixed effect models developed in the context of the research programme Forests and Climate Change to assess tree growth of Norway spruce, silver fir, European larch, Scots and mountain pine, European beech, Sycamore maple and common ash, and how this is influenced by growing in different mixed stands. More importantly, we identified how complementarity effects vary with site and stand conditions with interactions between site-dependent variables (e.g., stand density, stand development, temperature, moisture, soil pH) and the effect of mixing.

How are mixing effects modulated along resource gradients?

Results of the study showed that positive mixing effects increased with increasing drought conditions. This trend appear to be quite general for several species, but the degree of change is not the same for a given species growing in different mixtures, and there are even exceptions showing opposite relationship. This confirms that, in most of the cases, promoting mixed-species forests can be a suitable option to cope with climate change-induced impacts of increasing drought. However, we suggest that complementarity effects related to drought conditions should be investigated considering the species identity rather than using indices of species diversity, because the simple rule “the more species, the less the forest will be water-stressed” does not seem to work.

Temperature too was found to be responsible for altering complementarity for many of the investigated species. Under increasing temperature, complementarity mostly increased with increasing temperature, but the opposite also occurred. However, the effect of temperature might be positive or negative depending on the location of the mixed forest. Many other site-dependent factors such as soil acidity, nitrogen deposition, topographic features were associated with changes in complementarity effects.

READ MORE: The future of Silver fir under climate change and browsing

The magnitude of complementarity increased or decreased with increasing aboveground competition, stand density and stand development. In this case, however, a general trend across mixture types could not be identified. Showing that complementarity effects are not only contingent to climate but to stand characteristics is important in the context of adapting forest management to climate change because competition, stand development or density are properties can be directly modified through silvicultural interventions.

Working on this study, I quickly realized that disentangling mixing effects is quite complex because there are many, multiple factors involved. Once again I come to repeat that is impossible to draw a general conclusion for certain phenomenon. I am convinced that we certainly cannot say that species A grow better if associated with species B without adding “under this conditions”. This reflects the higher complexity of the mechanisms regulating tree growth, which cannot be simplified with simple relationships and averages.

We could show that thanks to quality observation data and robust statistical approaches it is possible to identify the conditions under which the promotion of mixed-species forests can also help in fostering forest productivity. Careful assessments at the level of species identity under changing resource availability are always needed, though.

This post is based on material from an article appeared in the Journal of Ecology blog: Diversity effects in temperate mixed forests are modulated by stand and environmental factors

Find here the full paper: Mina M, Huber MO, Forrester DI, Thürig E, Rohner B (2017) Multiple factors modulate tree growth complementarity in central European mixed forests. Journal of Ecology. First published: 14.09.2017

Main photo: Canopy closure in a mixed beech-spruce forest in the south-eastern Alps. Author of the photo: Marco Mina